The present invention relates to electrostatographic reproducing methods and apparatus and more particularly to methods and apparatus for enhanced reproduction of pictorial quality.
In the electrostatographic reproducing apparatus commonly in use today, a photoconductive insulating member is typically charged to uniform potential and thereafter exposed to a light image of an original document to be reproduced. The exposure discharges the photoconductive insulating surface in exposed or background areas and creates an electrostatic latent image on the member which corresponds to the image areas contained within the usual document. Subsequently, the electrostatic latent image on the photoconductive insulating surface is made visible by developing the image with developing powder referred to in the art as toner. Most development systems employ a developer material which comprises both charge carrier particles and charged toner particles which triboelectrically adhere to the carrier particles. During development, the toner particles are attracted from the carrier particles by the charge pattern of the image areas in the photoconductive insulating area to form a powder image on the photoconductive area. This image may subsequently be transferred to a support surface such as copy paper to which it may be permanently affixed by heating or by the application of pressure.
This process is basically a high contrast image process in that it is capable of the reproduction of line copy wherein toner is deposited in image areas and not deposited in non-image areas. Accordingly, it does not provide good reproduction of photographic images wherein there is a gradation of color, a gray scale throughout the image. This is in part due to the characteristics of the materials used as the photoconductive insulating layers during exposure in that exposure to a little light for a short period of time results in a fast discharge to a very low level of charge. As a result the electrostatic latent image so produced results in image areas of high charge and non-image areas of very low charge despite the fact that there may be several gradations of color in the original document being reproduced. This characteristic may be represented graphically by a photo-induced discharge curve which is a plot of photoconductor plate surface potential versus the log of exposure. If this curve has a relatively steep slope it means that the photoconductive insulating layer will discharge rapidly with a relatively small increment of light above the threshold at which the first detectable change in potential is detected. Accordingly, in order to be able to reproduce gradations of color, a flat or relatively small slope of this curve is desired which thereby provides more discriminating information as to the gradations of color in the original document that it is desired to reproduce. This range of exposure from black to white in a developed image, which is referred to as the dynamic range, is desired to be as long as possible to provide a more discriminating gradation of charge corresponding to gradation in light intensity which corresponds to gradation in the image.
The xerographic reproduction of material with graded tonal values such as continuous tone or screened pictures or other uniform solid areas may be obtained by breaking up the electrostatic latent image into a series of parallel lines or dots separated by discharged areas and thereby introduce fringing electrostatic fields which are of developable magnitude. Although the solid areas are broken up into a pattern of parallel lines or dots, they can have sufficiently close spacing so that the line structure is not readily discernible to the unaided eye. For example, after conventional charging and exposure the photoconductor is exposed a second time to a white dot pattern or a to a bar pattern of light having high contrast and well defined edges. As a result of the second exposure, the previously unexposed or partially exposed areas of the plate are discharged in the line or dot pattern. The exposed areas will be unchanged from the level to which they were discharged during the first exposure thus strong electric fields are set up which are developable by conventional methods. Screen patterns of a 100 to 200 lines per inch are typically used in this procedure. However, since this technique requires the use of a screen and a second exposure step the final print may be of reduced image density and sharpness.